1. Field of the Invention
This invention relates to a method of forming a substantially closed void, for example between two metal lines or structures on a substrate such as a semiconductor wafer.
2. Background of the Invention
It is well known in the semiconductor and nanotechnology industries that as device dimensions become smaller and the metal current carrying lines on such devices accordingly get closer together, problems arising from capacitive coupling between the metal lines increase. Over the past few years, therefore, there has been a drive to produce flowable gap-filling dielectric materials with low dielectric constants. However, it has long been recognised, that a vacuum void is the lowest constant dielectric material that can separate the lines and, for example, U.S. Pat. No. 4,920,639, which was granted back in 1990, discusses a system for achieving such an arrangement, based on the creation of such voids.
Such voids are frequently called “air gaps” in the literature). However, this approach presents its own problems, because a system of support pillars are required for the metal interconnects and the voids either have to be temporarily capped to prevent subsequent deposition filling them or the material in those “voids” has to be removed at the end of the fabrication process.
Another approach is to use non-conformal deposition of a material which does not properly fill small gaps, but this requires subsequent chemical mechanical polishing. Further approaches are shown in U.S. Pat. Nos. 5,324,683 and 6,091,149, but both involve the formation of a finer detail gap to allow the void to be finally bridged and as device manufacturer is close to the physical limits of lithography, the formation of such finer detail becomes impractical.
Similar issues can arise in the formation of semiconductor and dielectric, trenches and membrane structures in MEMS and transducers.